Dental resin composite is a composite material composed mainly of a polymerizable monomer, a polymerization initiator and a filler. Dental resin composite can be imparted a color equivalent to natural tooth color and is easy to handle in the restoration operation of tooth; therefore, it is in wide use as a material for tooth restoration recently.
The application site of dental resin composite is various and includes, for example, shallow cavity where esthetics is required, the whole portion of tooth crown to be restored, and deep cavity (e.g. root canal after pulpectomy) where the light of dental light applicator does not reach. Rebuilding of abutment tooth, for example, is conducted to the root canal after pulpectomy.
Of the above-mentioned restoration sites, the shallow cavity where esthetics is required, is a site which is exposed to gaze of other persons. In the restoration of such a site, there is ordinarily used a dental resin composite containing a photopolymerization initiator. The photopolymerization initiator allows polymerization to start upon light application. The photo-curing dental resin composite containing a photopolymerization initiator does not allow polymerization to start unless light application is made. Accordingly, light-curing dental resin composite, even when all components are mixed in a single paste, can be produced and stored under light shading.
In contrast, when a deep cavity (e.g. root canal after pulpectomy) is restored, no sufficient light reaches to the restoration site. As a result, the photopolymerization initiator does not function sufficiently and the dental resin composite is not polymerized sufficiently. Accordingly, the cured material obtained is inferior in various properties including mechanical strength.
In order to solve this problem, a so-called chemical polymerization initiator has been mainly compounded in dental resin composite. This chemical polymerization initiator is composed of two or more polymerization initiator components and, when these components are allowed to contact with each other, a polymerization-initiating species is generated (even at room temperature). Accordingly, a chemical-curing type dental resin composite is produced and stored being divided into two portions (ordinarily, two pastes). The dental resin composite stored being divided in two portions is mixed right before the use of the resin composite.
As explained above, the photo-curing type resin composite or the chemical-curing type resin composite is used depending upon the application purpose. However, there are many cases where a resin composite having both photo-curing characteristic and chemical-curing characteristic is convenient depending upon the application condition.
For example, there is a case where a resin composite is filled in a deep cavity (e.g. root canal after pulpectomy) and cured, an abutment for crown bridge is constructed using a curing material having the same formulation as the resin composite, and then the preparation form of abutment tooth is corrected using a grinding tool. In this case, a single resin composite is used easily, which shows good chemical-curing characteristic in a deep cavity and shows photo-curing characteristic so as to enable efficient and easy rebuilding of abutment. As the resin composite for such an application, there is a resin composite in which a photopolymerization initiator is compounded in a chemical-curing resin composite and which has both of chemical-curing characteristic and photo-curing characteristic (see, for example, Patent Literatures 1 and 2). This resin composite is so called “dual-cure resin composite”.
Next, the photopolymerization initiator compounded in a photo-curing resin composite is focused. In this case, there is required a photopolymerization initiator which allows for no curing under a weak light (e.g. an environmental light) and allows a resin composite to cure rapidly when a strong light is applied by, for example, a light applicator. As the photopolymerization initiator meeting this requirement, there is proposed a photopolymerization initiator system in which an α-diketone compound, an s-triazine compound having a trihalomethyl group as a substituent, and an aliphatic tertiary amine compound are combined (see, for example, Patent Literatures 3 and 4). This photopolymerization initiator has very high polymerization activity.
When this photopolymerization initiator is irradiated with a light, energy level of the α-diketone compound is excited, the excited energy of the α-diketone compound is transferred to the s-triazine compound, thereby a radical contributing to polymerization is generated. In this case, the co-presence of the aliphatic tertiary amine compound makes easy the presence of activated s-triazine compound. As a result, the amount of radical generated increases greatly, exhibiting high polymerization activity.
In the above photopolymerization initiator, the aliphatic tertiary amine compound imparts, to the s-triazine compound, higher polymerization activity than a primary amine compound or a secondary amine compound. Of various aliphatic tertiary amine compounds, an aliphatic tertiary amine compound in which at least two of the three saturated aliphatic groups bonding to the nitrogen atom have an electroattracting group as a substituent, imparts high storage stability to a resin composite and therefore is extremely useful (Patent Literature 4 ,paragraph 0030).
Further, it is known to use, as the polymerization initiator component, an aryliodonium salt, a sensitizer and an electron donor (e.g. Patent Literature 6).
Meanwhile, it is necessary to focus also on the chemical polymerization initiator contained in a chemical-curing type resin composite. As a chemical polymerization initiator showing a high activity, there is known a system in which an organic peroxide is reacted with an aromatic tertiary amine compound in which the nitrogen atom is directly bonded to the aromatic group, to generate a polymerization-activating species (e.g. Patent Literature 5, paragraph 0020).
An investigation by the present inventors revealed that when, in the above system of chemical polymerization initiator, an N,N-di(hydroxyalkyl)-p-toluidine compound is used as the aromatic tertiary amine compound in which the nitrogen atom is directly bonded to the aromatic group, the dental resin composite obtained shows a very favorable property. That is, it was revealed that, in using the dental resin composite obtained, there is an appropriate time from the timing of mixing the p-toluidine compound with the organic peroxide to the timing of generation of polymerization-initiating species and, once the polymerization-initiating species begins to generate, the generation amount increases rapidly and the polymerization is complete in a short time.
That there is an appropriate time (ordinarily 2 to 10 minutes) from the timing of mixing the organic peroxide with the amine compound to the timing of generation of polymerization-initiating species, means that, in dental treatment, a sufficient time can be secured in filling operation of resin composite. As a result, a dentist can conduct treatment with afford. Further, that once the polymerization-initiating species begins to generate, the generation amount increases rapidly, means that the resin composite filled can be cured in a short time.
Patent Literature 1: JP61-186303 A
Patent Literature 2: JP2005-170813 A
Patent Literature 3: JP2005-76022 A
Patent Literature 4: JP2005-89729 A
Patent Literature 5: JP7-196431 A
Patent Literature 6: JP63-273602 A